Implement full-completion insertion and acceptance flow

[FuJacob]

Problem

Accepting a full Compose result is different from accepting a small inline autocomplete chunk. The app needs to insert a larger block reliably, avoid stale-focus insertion, and make cancellation/pass-through behavior obvious.

Goal

Implement the insertion and acceptance foundation for full Compose results while preserving the current autocomplete acceptance path.

Proposed Scope

• Define how a Compose result is represented in active suggestion/session state.
• Insert the full result at the focused caret only when focus and context are still valid.
• Cancel pending Compose work on focus changes, app-disabled state, global disable, or mode changes.
• Decide whether the overlay previews the full result, a collapsed preview, or a summary.
• Ensure Tab pass-through still works when there is no valid Compose result.
• Add tests around stale focus, disabled state, and acceptance behavior.

Acceptance Criteria

• A full Compose result can be accepted into a supported focused text field.
• Stale results are not inserted after focus/app/context changes.
• Existing autocomplete chunk acceptance continues to work.
• Cancellation and disabled-state behavior follows the existing suggestion availability model.
• Tests cover valid insertion, stale insertion prevention, and pass-through behavior.

Open Questions

• Should long insertion use pasteboard, accessibility text insertion, synthetic key events, or an app-specific strategy?
• How should multiline drafts be previewed before acceptance?
• Should users be able to accept part of a Compose result later?

Parent: #66

[FuJacob]

Closing for OSS launch cleanup. Part of speculative Compose Mode epic (#66).